1. Field of the Invention
This invention relates to structural testing, and in particular, relates to a device used to excite structures during dynamic measurements.
2. Description of the Related Art
There are a number of different types of impact testing used to evaluate industrial and consumer products to determine the mechanical resonances and vibration amplitudes of the tested structures, which are indicative of the functional suitability of the structures. Such testing during the development of products, for example, cutting tools, allows detection of structural designs which are susceptible to vibrations at their mechanical resonances, and thus may be subject to failure, malfunction or safety problems. Such tests are common on products which are used at high speeds and which carry passengers, such as automobiles or aircraft. It is important that the excitement produced in the testing be easy to apply and direct, have a large enough bandwidth and a great enough force, and be well-measured.
Prior simple impact testing devices generally utilize either some type of shaker (electric and hydraulic) or an impact test hammer having a sensor portion and a striking portion for imparting a sharp, manual impact to the structure at the desired point. Shakers are fed with a time-varying signal, and are attached to the structures to be tested, and are often not convenient to use, are expensive, and require a rather high skill level in the user. Hammers are more convenient, but also require a high skill level in the user. The size of the hammer and the material in the tip control the range of excitation frequencies. For excitation of very small cutting tools, the hammer must be quite small. It is difficult to hit the small target and to hit it with the required force. Substantial training, experience, and dexterity are often required for satisfactory testing. If the target point is missed or hit at the incorrect force, there can be measurement errors. Such devices also require space for the hammer device to be swung prior to impacting the structure. For example, in the patent of Umemura et al. (U.S. Pat. No. 5,025,655), an impact test hammer has a hollow cylindrical handle with a sensor portion on one end to sense the striking force when applied to a test object, and a grip portion on the other end, which contains a circuit operatively connected to the sensor through the handle and includes a range switch, scale conversion amplifier and a wind amplifier.
In the impact testing apparatus of Garritano et al. (U.S. Pat. No. 4,640,120), a weighted dart is dropped from a prescribed height through a tubular guide to impact a test specimen when traveling at a desired velocity and level of kinetic energy. A force transducer at the tip of the dart provides impact force information as the tip penetrates the specimen. This apparatus requires that the portions of the specimen to be tested be accessible to testing from above.
In the patent of Adelman et al. (U.S. Pat. No. 4,682,490), a power-actuated instrument comprising a movable means in a housing and an electrically responsive pulse means for triggering movement of the movable means produces a single impact force of variable magnitude and electronic means for detecting the impact force delivered, including the mechanical resonances and vibration amplitudes of the structure. The disadvantage of this instrument is that in order to achieve sufficient force, the mass of the moving component must be large. In contrast, in impact testing, the mass of the xe2x80x9chammerxe2x80x9d should be small in comparison to the mass of the target. This instrument is also expensive to manufacture, bulky, and requires setting the distance from the xe2x80x9cgunxe2x80x9d to the target using some Teflon screws.
The apparatus for non-destructive testing of Evans (U.S. Pat. No. 4,519,245) has an impactor means for impacting the surface of a material with a predetermined force, which is regulated in magnitude and duration of the force by a control means, such as a solenoid connected to a current source and sink, and a sensor for detecting the response from the material after impact and for generating a signal proportional to the amplitude and frequency of the detected response, as compared with a stored reference signal.
Impact testing is often carried out by dropping test samples from high towers or using pressurized gases. For example, the impact test apparatus of Meir (U.S. Pat. No. 4,696,182) includes a barrel containing pressurized gas and a shuttle with a bore for carrying the test sample. A trigger element causes the shuttle to accelerate toward an open end of the barrel toward a target aligned with the open end of the barrel. Again, this method is difficult to use with small structures to be tested, or with structures requiring testing of areas on multiple sides of an object.
Although the components of the invention herein are not in themselves new, the device and method herein enabling application and measurement of the force are new.
It is therefore an object of the invention to provide an inexpensive explosive excitation device and method for impact excitation that provides a controlled, broad bandwidth, high force excitation to a structure.
It is a further object of the invention to provide a device which requires less skill to use than prior impact hammers and shakers.
It is a further object of the invention to provide a device which imparts a force which can be quite large, and has an accurately controllable excitation location.
It is a further object of the invention to provide a device which utilizes explosive excitation and enables determination of the force produced by the device.
It is a further object of the invention herein to provide a device which produces an impact of wide bandwidth (containing a broad range of frequencies), in order to excite structures that have important dynamic characteristics at high frequencies.
It is a further object of the invention to provide a device which can be used to measure dynamic characteristics of machine tools, for example, to predict, improve and/or explain cutting performance; to measure structural dynamics of aerospace, automotive or many other industrial components and assemblies; to monitor machine condition; and to characterize buildings, bridges and other infrastructures.
Other objects and advantages will be more fully apparent from the following disclosure and appended claims.
The invention herein is an explosive excitation device and method of use thereof for exciting a structure during dynamic measurements. The device of the invention includes an accelerometer and an explosive charge. The device is attached at one end to the structure to be excited. The explosive charge is mounted between the accelerometer and the target structure. When the explosive charge is ignited, the accelerometer is blown away from the device. Knowing the acceleration, measured by the accelerometer, and knowing the mass of the accelerometer, the force applied to the target structure can be calculated. The information on the force of input and the resulting motion of the target structure allows calculation of the structure""s dynamic characteristics.
Other objects and features of the inventions will be more fully apparent from the following disclosure and appended claims.